A retired British businessman is on his way to a Texas jail after he was extradited to the United States to face charges that he illegally exported weapons parts to Iran.
Christopher Tappin, who is 65 and lives in Kent, has been handcuffed and put on a plane with a seat between two US marshals.
His tearful wife Elaine accompanied him to the airport.
His lawyer described the goodbye as "distressing".
"Mr Tappin has left for America," Karen Todner wrote on Twitter.
"Was v distressing when he said goodbye. The extradition treaty is inhumane."
Tappin, who said he was full of trepidation about his fate, denies any wrongdoing and says he is the innocent victim of a sting operation by US agents.
He was extradited under a controversial treaty signed off in 2003 by both the US and UK governments, which allows American prosecutors to demand British suspects are handed over without their cases ever being tested in a British court.
Earlier, he labelled his treatment a "disgrace" as he arrived at Heathrow police station.
"I look to (Prime Minister David) Cameron to look after my rights and he has failed to do so," he said.
"I have no rights. Abu Qatada is walking the streets of London today and we cannot extradite him. He has more rights than I have.
"If I was a terrorist I would not be going to America. I think it's a shame, a disgrace.
"The Conservative government, while in opposition, promised to reform the law and they failed to do so and they've let me down, they've let you down, they've let the whole country down."
During his time at the police station his overnight bag - all Ms Todner said he was allowed to take - was searched and he was allowed to spend time with his wife.
Ms Todner said: "He has been, I think, strong for his wife. He is quite calm but I think he is just dismayed by it.
"I think he can't really quite take it all in, he can't really quite comprehend what is happening to him."
Tappin fought extradition for years, but both the UK and European courts said that he could be extradited.
He said earlier: "I was born in this country, I'm a taxpayer in this country, why can't you look after me? I need your protection.
"It's the one time in my life that I need your protection, yet you let me go into a brutal system which I don't know if I'll ever survive."
At Heathrow, he said he had packed some "personal stuff" including books by TV presenter Jeremy Clarkson and golfer Seve Ballesteros.
Tappin's flight was bound for El Paso, Texas. He is due to appear in court on Monday morning after spending the weekend in custody.
Tappin, who has been caring for his wife, who has chronic Churg-Strauss Syndrome, said he was "not very confident at all" about his case.
"I won't be given leave to get any of my witnesses who are based in the UK because they are not allowed to testify in America by virtue of the fact they do not allow video interrogation. They have to appear personally.
"Unfortunately they will not appear in America so I don't know. I have certainly got enough facts to support my case but without the witnesses, their testimony, it's going to be very difficult.
"If I wanted anything, it was to be tried in the UK, not in America, because the Americans have never had to produce one piece of evidence.
"All the evidence shown to the court so far has come from our side.
"They have not had to produce any evidence whatsoever. We believe there is no evidence... it's just an accusation.
"By virtue of an accusation they are allowed to extradite people from one country to another."
He said this "seems ridiculous and a disgrace to our country".
Legal commentator Joshua Rozenberg has said there is nothing Mr Cameron or any other politician can now do to stop Tappin's extradition going ahead.
"The extradition treaty between the US and UK doesn't require evidence to be presented in court, it simply provides for information to be sent from the requesting country to the country where that individual happens to be," he said.
"If that information satisfies the terms of the treaty, there's very little discretion for the Home Secretary , a minister, or the courts."
His lawyers will attempt to get him bail, but as a foreigner he is considered a flight risk and so may be remanded into custody.
In the longer term, Tappin and his legal advisers will have to weigh up whether to fight the allegations against him, which may drag on for years or come to some kind of plea-bargain deal with the authorities to speed up his return to the UK.
Either way, his stay in the American penal system will be arduous for a pensioner, according to Ms Todner.
"He'll be in a federal prison, which is different to a state prison, but he'll be in general population so there'll be no special area for him or anything like that," she said.
"Basically, he'll be in a dormitory of about 400 men, all facing serious criminal charges.
"He will not be in a low category prison because, as a foreigner, he's automatically declared a flight risk, so he's not entitled to go into what they call a 'prison camp', which is the lowest category."
Tappin is selling his family home to fund his legal fees in the US. If he fights his case and loses, he believes he could be sentenced to up to 35 years in jail, which would effectively mean he would die behind bars.
He knows the only realistic way he can get back to his family sooner is to plea bargain with the prosecutors.
Tappin is not the first Briton to find himself extradited to the US under the current treaty.
The so-called NatWest three were handed over to American prosecutors following the collapse of the Enron energy company and eventually had to cut a plea deal with the authorities to secure their freedom.
Other cases such as the computer hacker Gary McKinnon, who allegedly hacked into US military systems and is fighting his extradition, are still going through the UK courts.
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